We present the results of simulations, using density functional theory (DFT) with generalized gradient corrections (GGA), on the troilite (FeS), pyrrhotite (Fe1-xS) and MnP phases of FeS. The values obtained for the cell parameters and c/a ratio of troilite accurate to within 1% of those determined by experiment, a significant improvement on previous simulations. Energy-volume curves for FeS in the troilite and MnP structures indicate a pressure-induced transition at 4GPa (experimentally observed at 3.4 GPa). Comparison of spin-polarised and non-spin-polarised simulations of the troilite structure demonstrate the significance of magnetostructural effects in determining the c/a ratio and shed light on the magnetic and volume collapse of FeS on its transition from the MnP to a monoclinic structure at 6.7 GPa. Simulations of different (001) surface terminations of troilite indicate that stable surfaces are characterised by triangles of iron atoms "capped" with a sulphur atom.